Electric vehicles encompass vehicles in which the electrical energy necessary for their movement is stored in batteries and vehicles in which the electrical energy is produced on board, by an engine driving a generator or by a fuel cell. The traction of the vehicle is ensured by one or more electric machines. The braking of the vehicle is ensured by a conventional mechanical braking system. The prior art already contains very numerous electric-traction vehicle proposals. It is possible to cite for example U.S. Pat. No. 5,418,437 which describes a four-wheel vehicle, of series hybrid type, each wheel being driven by an electric machine specific to it, a controller making it possible to drive the wheel motors and ensuring the management of energy provision to the motors from an alternator or battery. This patent remains silent on the management of the electrical braking.
However, an electric machine being reversible, it can also be used as an electrical generator during the vehicle braking phases and in this case it transforms the mechanical braking energy into electrical energy that the vehicle must absorb, optionally by thermal dissipation. This mode of operation is often called “electrical braking” or “recovery braking”.
In practice, electric machines operate as generators to ensure moderate deceleration of the vehicle, to recover, as far as it is possible, the energy and store it in electrical accumulators, or even to dissipate it so as to lessen the degree to which the mechanical brakes of the vehicle are invoked. The main braking of a vehicle is in fact ensured by hydraulically controlled mechanical brakes, in general in an assisted manner, and now usually provided with an anti-lock function commonly denoted “ABS”. Braking is a paramount safety function on a vehicle. Mechanical brakes are of considerable power, capable of locking the wheel, power clipping being ensured by the anti-lock function, the clipping being related to the limit of grip. To ensure the safety of passengers, the braking system of a passenger vehicle is in general capable of ensuring a deceleration of the order of 1 “g”, g being the unit of acceleration for which the value “1” corresponds to terrestrial gravity.
Additionally, in an electric-traction vehicle, a particularly beneficial configuration is produced by integrating the electric machine into the wheel since this does away with mechanical shafts and offers more latitude for the general architecture of the vehicle. The prior art contains several arrangements for integrating electric machines into the wheels. Patent application WO 2003/065546 proposes to deploy four electric machines transmitting their torque to the wheel by means of an epicyclic train. Patent application EP 0878332 discloses a ground linkage which integrates at one and the same time the vertical suspension of the wheel inside the latter and a rotary electric-traction machine. There is a reduction stage between the wheel and the electric machine, the latter being meshed with a toothed wheel coaxial with the wheel. Of course the wheel comprises a disc brake so as to ensure the service braking function. Furthermore, the ground linkage comprises a pivot so as to make it possible to deflect the wheel. All the mechanical functions of a ground linkage are thus integrated into the wheel.
Operational safety being paramount, numerous traditional mechanical brake control systems have been proposed, such as for example that of patent application EP 1 026 060 which describes redundant means, driving by majority decision, several low-voltage power supplies of the control systems for maintaining total operability even if several batteries have failed. Let us also cite U.S. Pat. No. 6,244,675 describing a braking control whose position is measured by three sensors, supplied by two independent sources: one sensor is supplied by a first source, another by a second source, and the third by both sources via diodes; if one of the sources is out of service, two sensors are still supplied and remain in service.
It is also possible to cite U.S. Pat. No. 6,476,515 which shows a use of four sensors, measuring different physical quantities. The sensors are grouped as a function of the physical principle measured and they are all necessary for the normal calculation of the braking force.
The invention pertains to electrical braking systems of a road vehicle equipped with wheels which are linked in rotation each to at least one rotary electric machine, each rotary electric machine cooperating with a single wheel. In such an architecture, it is possible to accord the electrical braking a predominant role, both as regards power and control of the stability of the vehicle (functions known by the terms ABS and ESP) since it is possible to selectively control the wheel torque on each of the wheels via the driving of the rotary electric machine (machines) associated therewith. Again for this purpose the electrical braking must be extremely reliable.
The objective of the present invention is to improve the safety of the electrical braking systems for electric-traction vehicles. In particular, the objective is to propose an architecture of an electrical braking system such that it is possible to dispense with mechanical brakes and ensure the service braking function purely electrically. More specifically, the objective of the present invention is, through particular configurations of redundant means, to make such a system extremely safe in the detection and utilization of a braking request by the driver of a vehicle.